How to get the start time and end time in utc of a day for a specified timezone in javascript?

Question

As the title stated, I want to get the start time and end time of a day for certain timezone and convert them to utc time. Here is my part of my implementation:

//convert current local time to specified timezone time
var converted = moment.tz(moment(), timezone).format("YYYY-MM-DD");
var full_format = "YYYY-MM-DD HH:mm:ss";

// get start time and end time in the timezone
var start = converted + " 00:00:00";
var end = converted + " 23:59:59";

// how to convert them in utc time by momentjs or other methods? 
var utc_start_time = ?
var utc_end_time = ?

The question is how to convert the time in certain timezone to utc time. Or is there any other decent solutions to it? Thank you!

Edit:

I figured out a way to make it myself which is not quite decent.

var converted = moment.tz(moment(), timezone).format("YYYY-MM-DD");         
var full_format = "YYYY-MM-DD HH:mm:ss";
var start = converted + " 00:00:00";
var end = converted + " 23:59:59";
var utc_start_time = moment(start).add(utc_offset * -1, 'hours').format(full_format);
var utc_end_time = moment(end).add(utc_offset * -1, 'hours').format(full_format); 

Any suggestions on improvements are welcome. Thanks

Answer 1

Depending on what exactly you want to do:

// for the current day
var start = moment.tz(timezone).startOf('day').utc();
var end = moment.tz(timezone).endOf('day').utc();

// for a specific moment (m)
var start = m.clone().tz(timezone).startOf('day').utc();
var end = m.clone().tz(timezone).endOf('day').utc();

// for a specific calendar date
var m = moment.tz(date, format, timezone);
var start = m.clone().startOf('day').utc();
var end = m.clone().endOf('day').utc();

You can then format start and end however you like with the format function.

Keep in mind also that not every day starts at midnight in every time zone, nor do all local days have 24 hours.

Answer 2

I ran into this problem as well. Unfortunately, moment.js seems to be telling people to use other options. (I guess they are more focused on US politics judging by their website, ugh). Also, many of the momemt funtions in the accepted answer are now depricated. Luxon is the first library that moment suggest as an alternative.

let DateTime = luxon.DateTime;

var dt = DateTime.local() // get the current time in local timezone
  .startOf('day')         // set the time to the start of the current day
  .setZone('GMT');        // change time zone back to GMT (zero offset)
console.log('Start: ' + dt.toISO());  // UTC equivalent for local start of day

var dt = DateTime.local() // get the current time in local timezone
  .endOf('day')           // set the time to the end of the current day
  .setZone('GMT');        // change time zone back to GMT (zero offset)
console.log('End  : ' + dt.toISO());  // UTC equivalent for local end of day (1 ms before midnight)

<script src="https://cdn.jsdelivr.net/npm/luxon@1.22.2/build/global/luxon.min.js"></script>

Answer 3

Here is a native solution that uses Intl.DateTimeFormat to handle timezones.

It can find the begining/end of the day with specified timezone.

To convert a Date instance to UTC time, use Date.prototype.toISOString.

const beginingOfDay = (options = {}) => {
  const { date = new Date(), timeZone } = options;
  const parts = Intl.DateTimeFormat("en-US", {
    timeZone,
    hourCycle: "h23",
    hour: "numeric",
    minute: "numeric",
    second: "numeric",
  }).formatToParts(date);
  const hour = parseInt(parts.find((i) => i.type === "hour").value);
  const minute = parseInt(parts.find((i) => i.type === "minute").value);
  const second = parseInt(parts.find((i) => i.type === "second").value);
  return new Date(
    1000 *
      Math.floor(
        (date - hour * 3600000 - minute * 60000 - second * 1000) / 1000
      )
  );
};

const endOfDay = (...args) =>
  new Date(beginingOfDay(...args).getTime() + 86399999);

const beginingOfYear = () => {};

console.log(beginingOfDay({ timeZone: "GMT" }));
console.log(endOfDay({ timeZone: "GMT" }));
console.log(beginingOfDay({ timeZone: "Asia/Tokyo" }));
console.log(endOfDay({ timeZone: "Asia/Tokyo" }));